Counters

Tom Kelliher, CS 240

Apr. 25, 2012

Administrivia

Announcements

Assignment

From Last Time

Shift registers.

Outline

1. Ripple counters.

2. Synchronous counters.

3. VHDL.

Coming Up

Memory.

Ripple Counters

The increment ripples -- propagation delay problems.

Slow counters.

Basic idea:

1. Each flip-flop's !Q is fed back to D. What does this accomplish?

2. Flip-flop $i$'s !Q is used to clock flip-flop $i + 1$. What does this accomplish?

3. Trace the propagation delay of the clock if the count is currently 1111 and a rising clock edge is applied.

4. Through what sequence, starting with 0000, does the counter count?

Synchronous Counters

1. All flip-flops receive same clock signal.

2. Still have some rippling. (Where?)

3. Inputs: clk, enable.

4. Outputs: count, carry output (for cascading).

5. After state table minimization, input equation for bit $i$:
   
   $$C_i \oplus (C_0 \cdot C_1 \cdot \ldots \cdot C_{i-1} \cdot {EN})$$
   
   

6. One bit slice for serial gating:
   

7. Serial vs. parallel gating.

VHDL

32 bit up counter with enable and reset.

-- Up counter with enable and reset
--
-- Note how en must be handled after the flip-flop generating
-- code.

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;

entity counter is
  
  port (
    clk, reset_n, en : in  std_logic;
    q                : out std_logic_vector (31 downto 0);
    co               : out std_logic);

end counter;

architecture behavioral of counter is

  signal count : std_logic_vector (31 downto 0);
  
begin  -- behavioral

  q <= count;

  state: process (clk, reset_n)
  begin  -- process state
    if reset_n = '0' then
      count <= X"00000000";
    elsif clk'event and clk = '1' then
      if en = '1' then
        count <= count + X"00000001";
      end if;
    end if;
  end process state;

  carry_out: process (count, en)
  begin  -- process carry_out
    if count = X"FFFFFFFF" and en = '1' then
      co <= '1';
    else
      co <= '0';
    end if;
  end process carry_out;
  
end behavioral;

Tom Kelliher